Method for forming patterned doping regions

ABSTRACT

A method for forming doping regions is disclosed, including providing a substrate, forming a first-type doping material on the substrate and forming a second-type doping material on the substrate, wherein the first-type doping material is separated from the second-type doping material by a gap; forming a covering layer to cover the substrate, the first-type doping material and the second-type doping material; and performing a thermal diffusion process to diffuse the first-type doping material and the second-type doping material into the substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.13/710,795, filed on Dec. 11, 2012, which claims priority of TaiwanPatent Application No. 101135229, filed on Sep. 26, 2012, the entiretiesof which are incorporated by reference herein.

BACKGROUND

1. Technical Field

The technical field relates to a method for forming patterned dopingregions, and more particularly to a method for forming patterned dopingregions of a solar cell.

2. Description of the Related Art

In recent years, due to rise of environmental protection and globalwarming, the green industry has been greatly developed. Solar cells area major technology in the green industry. Methods for developing a highefficiency, low-cost solar cell have become an important focus ofresearch. Solar cells can be disposed on buildings, such as houses, andmovable apparatus, such as cars; indoors, or on portable electricdevices, to convert light into electrical power.

The conventional art develops solar cells with selective emitters.Emitters with a low concentration between electrodes can reduce therecombination of carriers at the surface of the cell, while emitterswith a higher doping concentration under electrodes can provide goodcontact. Therefore, compared to conventional solar cells with emittingstructures that have a constant doping concentration, solar cells withselective emitters have higher open loop voltage (Voc) and short-circuitcurrent (Isc) and thus higher photoelectric conversion efficiency.

In 2010, Sunpower Company developed a solar cell with a back electrodehaving a cross-finger shape and an efficiency of 24.2%. Although thissolar cell has a good conversion efficiency, it has a very highmanufacturing cost due to its requiring many high-temperature andphotolithography processes.

Both the solar cells with selective emitters and the solar cells with aback electrode having a cross-finger shape employ patterned dopingregions to increase conversion efficiency. However, the process forforming patterned doping regions is more complicated and requires ahighly accurate process, such as lithography, thus increasing the costof the solar cell.

SUMMARY

The present disclosure provides a method for forming doping regions,comprising providing a substrate, forming a first-type doping materialon the substrate, forming a second-type doping material on thesubstrate, wherein the first-type doping material is separated from thesecond-type doping material by a gap, forming a covering layer to coverthe substrate, the first-type doping material, and the second-typedoping material, and performing a thermal diffusion process to diffusethe first-type doping material and the second-type doping material intothe substrate.

Another embodiment of the disclosure provides a method for formingdoping regions, comprising providing a substrate, forming a dopingmaterial on the substrate, forming at least one patterned covering layeron a portion of a surface of the doping material, wherein the portion ofthe doping material is covered by at least one patterned covering layer,and another portion of the doping material not covered by the patternedcovering layer is exposed, and performing a thermal diffusion process todiffuse the doping material into the substrate, forming a first dopingregion in a portion of the substrate underlying the patterned coveringlayer and forming a second doping region in a portion of the substratenot covered by the patterned covering layer, wherein the dopingconcentration of the first doping region is greater than the dopingconcentration of the second doping region.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein,

FIG. 1A˜FIG. 1G show cross sections of intermediate stages of a methodfor forming a structure comprising p-type regions and n-type regions.

FIG. 2A˜FIG. 2D show cross sections of intermediate stages of a methodfor forming a structure comprising p-type or n-type patterned dopingregions.

FIG. 3A shows curves with time as a function of temperature for variousexamples of the disclosure.

FIG. 3B shows resistance for examples under various heating conditions.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation,numerous specific details are set forth in order to provide a throughunderstanding of the disclosed embodiments. It will be apparent,however, that one or more embodiments may be practiced without thesespecific details. In other instances, well-known structures and devicesare schematically shown in order to simplify the drawing.

An embodiment of the disclosure forms p-type or n-type patterned dopingregions using a coating of phosphoric acid or boric acid. The embodimentuses screen printing to take off photolithography and further uses acoating layer to avoid the spread of coated phosphoric acid or boricacid. An embodiment of the disclosure can use a single thermal diffusingstep to get a p-type pattern region or an n-type pattern region.

A method for forming a structure comprising p-type regions and n-typeregions is illustrated FIG. 1A˜FIG. 1G. In an embodiment of thedisclosure, the p-type and n-type pattern doping regions are used toform the cross-finger back electrodes of a solar cell. First, referringto FIG. 1A, a substrate 102 is provided. The substrate 102 may comprisesilicon, gallium arsenide, gallium nitride, strained silicon, silicongermanium, silicon carbide, diamond, an epitaxy layer, and/or othermaterials. Next, a first mask 104 comprising first openings 106 isprovided overlying the substrate 102. In an embodiment of thedisclosure, the first mask 104 can be a screen. Thereafter, a first-typedoping material 108 is formed into the first openings 106 in the firstmask 104. The method for forming the first-type doping material 108 canbe spray coating, spin coating, or screen printing. In an embodiment inwhich the first-type doping material 108 is p-type doping material, thefirst-type doping material 108 can be a boride, aluminide or gallide.Furthermore, the first-type doping material 108 can be boron glass, andcan be formed by the following steps. Boric acid is coated on the firstmask 104, such as a screen, and is filled into the first opening 106.Next, a thermal process is performed to convert the liquid-state boricacid to a colloidal-state or a solid-state compound, such as acolloidal-state or a solid-state boride. The thermal process can have atemperature of 200° C.˜600° C. and can further have a temperature of250° C.˜350° C. In an embodiment in which the first-type doping material108 is n-type doping material, the first-type doping material 108 can bea phosphide, arsenide or telluride. Furthermore, the first-type dopingmaterial 108 can be phosphorous glass, and can be formed by thefollowing steps. Phosphorous acid is coated on the first mask 104, suchas a screen, and is filled into the first opening 106. Next, a thermalprocess is performed to convert the liquid-state phosphoric acid to acolloidal-state or a solid-state compound, such as a colloidal-state ora solid-state phosphide. The thermal process can have a temperature of200° C.˜600° C. and can further have a temperature of 250° C.˜350° C.

Referring to FIG. 1C, a second mask layer 110 comprising second openings111 is formed overlying the substrate 102, wherein the second mask 110can be a screen. Thereafter, a second-type doping material 112 is formedinto the second opening 111 in the second mask 110, wherein the methodfor forming the second-type doping material 112 can be spray coating,spin coating or screen printing. In an embodiment of the disclosure, thefirst-type doping material 108 can be p-type doping material, and thesecond-type doping material 112 can be n-type doping material. Inanother embodiment of the disclosure, the first-type doping material 108can be n-type doping material, and the second-type doping material 112can be p-type doping material.

In an embodiment in which the second-type doping material 112 is n-typedoping material, the second-type doping material 112 can be a phosphide,arsenide or telluride. Furthermore, the second-type doping material 112can be phosphorous glass, and can be formed by the following steps.Phosphorous acid is coated on the second mask 110, such as a screen, andis filled into the second opening 111. Next, a thermal process isperformed to convert the liquid-state phosphoric acid to acolloidal-state or a solid-state compound, such as a colloidal-state ora solid-state phosphide. The thermal process can have a temperature of200° C.˜600° C. and can further have a temperature of 250° C.˜350° C. Inan embodiment in which the second-type doping material 112 is p-typedoping material, the second-type doping material 112 can be a boride,aluminide or gallide. Furthermore, the second-type doping material 112can be boron glass, and can be formed by the following steps. Boric acidis coated on the second mask 110, such as a screen, and is filled intothe second opening 111. Next, a thermal process is performed to convertthe liquid-state boric acid to a colloidal-state or a solid-statecompound, such as a colloidal-state or a solid-state boride. The thermalprocess can have a temperature of 200° C.˜600° C. and can further have atemperature of 250° C.˜350° C.

Next, referring to FIG. 1D, the second mask 110 is removed. In anembodiment of the disclosure, the first-type doping material 108 and thesecond-type doping material 112 are separated by a distance d, and thedistance d can be 5 μm˜30 μm.

Referring to FIG. 1E, a covering layer 114 is formed on the substrate102, the first-type doping material 108, and the second-type dopingmaterial 112. The method for forming the covering layer 114 can be spraycoating, spin coating, screen printing, plasma enhanced chemical vapordeposition (PECVD), or atomic layer deposition (ALD). In an embodimentof the disclosure, the covering layer 114 can be SiN_(x) or Al₂O₃. Al₂O₃is a preferable material for the covering layer 114 since Al₂O₃ does notsubstantially react with boron doping material or phosphorous dopingmaterial, and Al₂O₃ is stable at high temperatures and is easilyremoved. Furthermore, Al₂O₃ does not substantially affect the diffusionof boron doping material and phosphorous doping material into thesubstrate. Compared to silicon oxide, using Al₂O₃ as the covering layerin the embodiment can avoid affecting the diffusion of boron orphosphorous.

Next, referring to FIG. 1F, a thermal diffusion process is performed todiffuse the first-type doping material 108 into the substrate 102 toform a first doping region 116, and diffuse the second-type dopingmaterial 112 into the substrate 102 to form a second doping region 118.In an embodiment of the disclosure, the temperature of the thermaldiffusion process can be 800° C. to 1000° C., and further can be 700° C.to 1200° C. It is noted that since the first-type doping material 108,the second-type doping material 112, and the substrate 102 are coveredby the covering layer 114 in the embodiment, the lateral diffusion ofthe first-type doping material 108 and the second-type doping material112 is limited. Therefore, the first-type doping material 108 and thesecond-type doping material 112 substantially diffuse downward toincrease the accuracy of the first doping region 116 and the seconddoping region 118. Thereafter, referring to FIG. 1G, the covering layer114, the first-type doping material 108 and the second-type dopingmaterial 112 are removed. In an embodiment of the disclosure, removingthe covering layer 114, the first-type doping material 108 and thesecond-type doping material 112 can be accomplished by immersion in afluorine-containing solution.

A method for forming a structure comprising p-type or n-type patterneddoping regions is illustrated FIG. 2A˜FIG. 2D. In an embodiment of thedisclosure, the p-type regions or n-type patterned doping regions areused to form selective emitters of a solar cell. First, referring toFIG. 2A, a substrate 202 is provided. The substrate 202 may comprisesilicon, gallium arsenide, gallium nitride, strained silicon, silicongermanium, silicon carbide, diamond, an epitaxy layer, and/or othermaterials. Next, a doping material 204 is formed on the substrate 202.In an embodiment in which the doping material 204 is p-type dopingmaterial, the doping material can be a boride, aluminide or gallide.Furthermore, the doping material 204 can be boron glass, and can beformed by the following steps. Boric acid is coated on the substrate.Next, a thermal process is performed to convert the liquid-state boricacid to a solid-state compound, such as a solid-state boride. In anembodiment in which the doping material 304 is n-type doping material,the doping material can be a phosphide, arsenide or telluride.Furthermore, the doping material can be phosphorous glass, and can beformed by the following steps: Phosphorous acid is coated on thesubstrate, and next a thermal process is performed to convert theliquid-state phosphoric acid to a solid-state compound, such as a solidstate phosphide. Thereafter, referring to FIG. 2B, a patterned coveringlayer 206 is formed on the doping material 204, wherein the patternedcovering layer 206 can be formed by screen printing, and can be SiN_(x)or Al₂O₃. Al₂O₃ is a preferable material for the patterned coveringlayer 206 since Al₂O₃ does not react substantially with the dopingmaterial 204, and does not substantially affect the diffusion of thedoping material 204.

Thereafter, referring to FIG. 2C, a thermal process is performed todiffuse the doping material 204 into the substrate 202, forming a firstdoping region 208 in a portion of the substrate 202 under the patternedcovering layer 206 and forming a second doping region 210 in a portionof the substrate 202 uncovered by the patterned covering layer 206. Itis noted that since the doping material 204 under the patterned coveringlayer 206 is covered, the doping material 204 does not evaporate todiffuse upward, thus the major portion of the doping material 204covered by the patterned covering layer 206 diffuse downward, but thedoping material 204 not covered by the patterned covering layer 206 canevaporate to diffuse upward. Therefore, the first doping region 208under the patterned covering layer 206 has a higher doping concentrationand lower resistance than the second doping region 210 uncovered by thepatterned covering layer 206. Therefore, the embodiment can form thefirst doping region 208 and the second doping region 210 with differentdoping concentrations and resistance using a single thermal diffusionprocess. Next, referring to FIG. 2D, the doping material 204 is removed.In an embodiment of the disclosure, the doping material 204 can beremoved by doping a solution containing fluorine.

Accordingly, with the formation of the patterned covering layer 206 onthe doping material 204, the embodiment can form the first doping region208 and the second doping region 210 with a different dopingconcentration using a single thermal diffusion process. The relationsbetween condition of thermal diffusion and resistance of the firstdoping region 208 and the second doping region 210 are illustrated inaccordance with FIG. 3A and FIG. 3B. FIG. 3A shows curves with time as afunction of temperature for various examples of the disclosure. FIG. 3Bshows resistance of the examples under various heating conditions. InFIG. 3A and FIG. 3B, A0 corresponds to resistance of the first dopingregion covered with a patterned covering layer. A1 corresponds toresistance of the second doping region uncovered with a patternedcovering layer and with a heating condition to raise the temperature to875° C. in one minute. A3.5 corresponds to resistance of the seconddoping region uncovered with a patterned covering layer and with aheating condition to raise the temperature to 875° C. in 3.5 minutes. A5corresponds to resistance of the second doping region uncovered with apatterned covering layer and with a heating condition to raise thetemperature to 875° C. in 5 minutes. A7 corresponds to resistance of thesecond doping region uncovered with a patterned covering layer and witha heating condition to raise the temperature to 875° C. in 7 minutes.According to FIGS. 3A and 3B, the second doping region covered with apatterned covering layer has the lowest resistance, and the conditionhas a longer rising time to 875° C., presenting a higher resistance.According to the results from FIG. 3A and FIG. 3B, the embodiment canfine tune the duration to the rise in temperature to a degree, such as875° C., for the first doping region and the second doping region toachieve a predetermined resistance. In addition, the disclosure canfurther adjust the thickness and concentration of the doping material totune the concentration of the first doping region and the second dopingregion.

While the disclosure has been described by way of example and in termsof the embodiments, it is to be understood that the disclosure is notlimited to the disclosed embodiments. It is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

What is claimed is:
 1. A method for forming doping regions, comprising:providing a substrate; forming a doping material on the substrate;forming at least one patterned covering layer on a portion of thesurface of the doping material, wherein the portion of the dopingmaterial is covered by the at least one patterned covering layer, andanother portion of the doping material not covered by the at least onepatterned covering layer is exposed; and performing a thermal diffusionprocess to diffuse the doping material into the substrate, forming afirst doping region in a portion of the substrate underlying the atleast one patterned covering layer and forming a second doping region ina portion of the substrate not covered by the at least one patternedcovering layer, wherein the doping concentration of the first dopingregion is greater than the doping concentration of the second dopingregion.
 2. The method for forming doping regions as claimed in claim 1,wherein the at least one patterned covering layer is SiN_(x) or Al₂O₃.3. The method for forming doping regions as claimed in claim 1, whereinthe doping material is a boride, aluminide, gallide, phosphide,arsenide, or telluride.
 4. The method for forming doping regions asclaimed in claim 3, wherein the step of forming the doping material onthe substrate comprises: coating a doping material on a substrate; andperforming a thermal process for the doping material to convert to asolid-state compound.
 5. The method for forming doping regions asclaimed in claim 1, wherein the step of forming the at least onepatterned covering layer comprises screen printing or spray coating.